LoopVectorizer: A reduction that has multiple uses of the reduction value is not

a reduction.

Really. Under certain circumstances (the use list of an instruction has to be
set up right - hence the extra pass in the test case) we would not recognize
when a value in a potential reduction cycle was used multiple times by the
reduction cycle.

Fixes PR18526.
radar://15851149

llvm-svn: 199570